1. Field
The present disclosure generally relates to antennas and, in particular, relates to dual-band antennas using high/low efficiency feed horns for optimal radiation patterns.
2. Description of the Related Art
When communicating between widely separated locations where the time that it takes for a signal to travel the intervening distance is significant, one common approach to improving the bandwidth of the communication link is to use different frequencies for the signals traveling in each direction. This allows signals to be sent continuously in both directions without interference. Each frequency is actually a frequency band, with a bandwidth determined in part by whether the signal is a frequency modulation, requiring more bandwidth, and the practical frequency sensitivity of the transmitter and receivers. It can be advantageous to have a wide separation between the frequencies of these two communication bands. For a two-band system, the bandwidth ratio (BWR) is defined as the ratio of the highest frequency of the high band to the lowest frequency of the low band.
Multiple-beam antenna systems are increasingly being used for satellite communications. For example, multiple-beam antennas are currently being used for direct-broadcast satellites (DBS), personal communication satellites (PCS), military communication satellites, and high-speed Internet applications. These antennas provide mostly contiguous coverage over a specified field of view on Earth by using high-gain multiple spot beams for downlink (satellite-to-ground) and uplink (ground-to-satellite) coverage.
An antenna may be considered as a transformer that matches the impedance of a transmission line to the impedance of free space, 377 ohms. Microwaves are electromagnetic waves with frequencies in the range of 300 MHz-300 GHz. A common microwave transmission line is a hollow line with a diameter of greater than a half wavelength and less than a full wavelength for the frequency of the signal that it carries. If a waveguide is left open-ended, the impedance of the line is not matched to that of free space and there is little gain. If the diameter of the waveguide is slowly expanded to a larger aperture, however, more gain can be realized while preventing undesired modes from reaching the waveguide. A funnel-like expansion of a circular waveguide is called a conical horn. A horn such as this is frequently used as the feed to a reflector antenna which shapes and steers the microwave beam or, for reception, collects a beam and feeds the microwaves into the horn.
The boundary conditions of a horn, including both the surfaces and discontinuities, may generate transverse modes for the electromagnetic field at the frequency of interest for the horn. Higher-level Transverse Electric (TE) fields tend to enhance the efficiency of a horn while Transverse Magnetic (TM) modes tend to reduce the efficiency. The mode numbers are usually indicated by suffix numbers such as TE11, TE12, etc., where multiple modes are referred to by use of “TE1,m” or “TM1,n” nomenclature.
Communication bands have been defined at many frequencies. Common bands include 12, 20, 25, 45, and 60 GHz bands. 45 GHz is a band commonly used by the military. Common combinations of frequency bands for bidirectional communication include 20 and 60 GHz (BWR=62/18=3.67) and 12 and 45 GHz (BWR=45.5/12.0=3.79).
Conventional multiple-beam satellite payloads employ separate uplink and downlink antenna suites. For example, the Anik-F2 satellite uses 5 uplink antennas in one antenna suite and 5 downlink antennas in another antenna suite, requiring 10 apertures. This is due to the lack of feed horns that can efficiently support both uplink and downlink frequencies that are widely separated. Each feed horn in the downlink antenna suit is capable of providing signal transmission over a selected transmission frequency band, whereas each feed horn in the uplink antenna suit is configured to provide signal reception over a required reception frequency band. These conventional multibeam satellites require several antenna apertures which consume valuable space on the spacecraft and are relatively expensive due to twice the number of reflectors and twice the number of feed horns required when compared to the dual-band antenna system disclosed herein.
Other conventional multiple-beam satellite payloads, such as AMC-15, AMC-16 and Rainbow, employ dual-band antennas using low-efficiency corrugated feed horns to realize dual-band operation, but have a significantly lower RF performance. Other conventional designs for a dual-band antenna may employ a frequency selective surface (FSS) subreflector, a low-frequency feed horn, a high-frequency feed horn, and a main reflector. The FSS subreflector employs resonant elements that are transparent to low frequencies and are reflective to high-frequency signals. Disadvantages with this approach include increased losses, the requirement of two separate feeds, a FSS subreflector, the complexity and consequent cost of the antenna, and narrow bandwidths.
Another design for a dual-band antenna involves the use of a coaxial feed horn, wherein the central horn works at the high-frequency band using waveguide modes and the outer horn works in the lower-frequency band in the coaxial mode. Disadvantages with this approach include high cross-polar levels due to coaxial modes, strong mutual coupling of signals between low and high frequency bands, and narrower bandwidth of operation.